When a fluorinated polymer such as polytetrafluoroethylene (hereinafter referred to as PTFE), a melt-moldable type fluororesin or a fluoroelastomer is formed by using an emulsion polymerization method, a fluorinated emulsifier is usually used since it does not hinder the polymerization reaction of a fluorinated monomer by chain transfer in an aqueous medium. Ammonium perfluorooctanoate (hereinafter referred to as APFO) is usually used as the fluorinated emulsifier.
An aqueous PTFE emulsion is obtained by emulsion polymerization of tetrafluoroethylene (hereinafter referred to as TFE). The aqueous emulsion is coagulated and dried to produce a fine powder of PTFE. The fine powder is shaped by a method such as paste extrusion and then used for various applications. On the other hand, an aqueous PTFE dispersion obtained by having the aqueous PTFE emulsion treated for stability or concentrated, as the case requires, is used for various coating or impregnation applications by adding various additives.
However, the fluorinated emulsifier is expensive, and the amount of its use is substantially influential over the production cost of the fluorinated polymer. Further, the fluorinated emulsifier is a substance which is not decomposable in nature, and accordingly, in recent years, it is desired to reduce not only waste water from a plant, but also the amount of a fluorinated emulsifier contained in an aqueous fluorinated polymer dispersion obtained.
In 2005, the U.S.A. Fluoropolymers Manufacturing Group of Society of Plastics Industries submitted to the U.S.A. Environmental Protection Agency a proposal for 90% reduction of APFO content in aqueous fluorinated polymer dispersion products relative to in 2000. As the reduction method, a method was proposed wherein an aqueous fluorinated polymer dispersion is contacted with a basic anion-exchange resin to adsorb and remove APFO.
For example, a process for producing an aqueous fluorinated polymer dispersion having a reduced content of a fluorinated emulsifier is proposed (Patent Documents 1, 2, 3, 4, 5 and 6), wherein a nonionic surfactant is added to an aqueous fluorinated polymer emulsion to produce an aqueous fluorinated polymer dispersion. Then, the aqueous fluorinated polymer dispersion is contacted with a basic anion-exchange resin to adsorb and remove a fluorinated emulsifier. Then, the aqueous dispersion is separated from the basic anion-exchange resin.
In the above conventional technique, a strongly basic anion-exchange resin is mainly used. It is said that by using the strongly basic anion-exchange resin, a fluorinated emulsifier can be adsorbed and removed efficiently from an aqueous fluorinated polymer dispersion as compared with the use of a weakly basic anion-exchange resin. However, if the strongly basic anion-exchange resin is used, the aqueous fluorinated polymer dispersion becomes easily coagulated and it is necessary to pay attention on the concentration control of a nonionic surfactant for improving the stability.
Further, since the counter ions of functional groups in the strongly basic anion-exchange resin are chlorine ions, it is concerned that the chlorine ions might be included in the aqueous fluorinated polymer dispersion, or that pipes might be corroded. Though it is possible to change the counter ions to hydroxyl ions (OH− ions) by preliminarily treating the strongly basic anion-exchange resin with an aqueous alkaline solution, it is then necessary to install a new production facility and the cost becomes high. Further, it is described that the counter ion is exchanged by a fluoride or oxalate in order to increase the efficiency of adsorption (Patent Documents 1, 2 and 4), but such is disadvantageous from the viewpoint of the cost performance.
Furthermore, when the aqueous fluorinated polymer dispersion is passed through a column packed with the strongly basic anion-exchange resin, the pH generally tends to be strongly alkaline after the passing therethrough. As a result, the decomposition of a nonionic surfactant is induced, and discoloration is likely to occur. Therefore, it is proposed to sufficiently lower the pH before the passing therethrough (Patent Document 6). Further, when the counter ions of the strongly basic anion-exchange resin are changed to the hydroxyl ions, the alkalinity becomes stronger and the resin itself becomes easy to decompose. Further, when the fluorinated emulsifier is eluted and recovered by using the strongly basic anion-exchange resin, the recovery efficiency is generally low as compared with the use of a weakly basic anion-exchange resin.
On the other hand, it is disadvantageous that a weakly basic anion-exchange resin is required to be used in a larger amount than the strongly basic anion-exchange resin during the recovery of the fluorinated emulsifier. However, it has been found that when the aqueous fluorinated polymer dispersion is contacted with the weakly basic anion-exchange resin, by an artifice for operation such as connecting many columns and changing sequentially saturated columns, the efficiency for removal of the fluorinated emulsifier is improved. However, it has been found that the efficiency for adsorption and removal of the fluorinated emulsifier tends to be low in a case where the molecular weight of the fluorinated polymer is high, or the concentration of the fluorinated emulsifier is high in the aqueous fluorinated polymer dispersion.
Accordingly, it is desired to develop a process for removing a fluorinated emulsifier efficiently from an aqueous fluorinated polymer dispersion, and producing an aqueous fluorinated polymer dispersion having a reduced content of a fluorinated emulsifier.
Patent Document 1: JP-A-2002-532583
Patent Document 2: JP-A-2005-501956
Patent Document 3: JP-A-2006-515375
Patent Document 4: WO2004/078836
Patent Document 5: WO2006/086793
Patent Document 6: WO2006/086795